This proposal seeks to investigate the feasibility of genetic repair of HbS mutations by a process of gene conversion. Dr. Kmiec and colleagues have developed an experimental strategy that centers around site-specific conversion of single base mutations using chimeric DNA-RNA oligonucleotides. This synthetic molecule folds into a double hairpin configuration that improves its stability in serum and in cells. Constructs designed to alter the HbS mutation have been found to efficiently catalyze gene conversion in lymphoblastoid cells. A similar strategy has now been shown to work in a variety of other cells, and with the alkaline phosphatase genes, the genes responsible for Crigler Najjar disease factor XIII, and factor IX deficiency. In other preliminary data, HbA to HBS conversion has been demonstrated in human CD34+ stem cells. In the proposed studies, Dr. Kmiec will explore the genotypic and phenotypic consequences of targeted gene conversion, the factors that determine conversion efficiency including the methods of delivery, and the levels of random mutagenesis produced by such constructs. Reconstitution studies using gene converted human stem cells will also be carried out in SCID/Beige mice. The long term goal of these studies is to enable gene conversion as a feasible option for gene therapy of hemoglobinopathies and other defined genetically-inherited diseases.